1. Field
This disclosure relates to radio frequency filters using surface acoustic wave (SAW) resonators, and specifically to filters and duplexers incorporating SAW resonators to provide very high rejection or isolation in a predetermined frequency band.
2. Description of the Related Art
As shown in FIG. 1, a SAW resonator 100 may be formed by thin film conductor patterns formed on a surface of a substrate 105 made of a piezoelectric material such as quartz, lithium niobate, lithium tantalate, or lanthanum gallium silicate. A first inter-digital transducer (IDT) 110 may include a plurality of parallel conductors. A radio frequency or microwave signal applied to the first IDT 110 via an input terminal IN may generate an acoustic wave on the surface of the substrate 105. As shown in FIG. 1, the surface acoustic wave will propagate in the left-right direction. A second IDT 120 may convert the acoustic wave back into a radio frequency or microwave signal at an output terminal OUT. The conductors of the second IDT 120 may be interleaved with the conductors of the first IDT 110 as shown. In other SAW resonator configurations (not shown), the conductors forming the second IDT may be disposed on the surface of the substrate 105 adjacent to, or separated from, the conductors forming the first IDT.
The electrical coupling between the first IDT 110 and the second IDT 120 may be frequency-dependent. The electrical coupling between the first IDT 110 and the second IDT 120 typically exhibits both a resonance (where the impedance between the first and second IDTs is very high) and an anti-resonance (where the impedance between the first and second IDTs approaches zero). The frequencies of the resonance and the anti-resonance are determined primarily by the pitch and orientation of the interdigitated conductors, the choice of substrate material, and the crystallographic orientation of the substrate material.
Grating reflectors 130, 132 may be disposed on the substrate to confine most of the energy of the acoustic waves to the area of the substrate occupied by the first and second IDTs 110, 120. However a portion of the energy of the acoustic wave, represented by the dashed arrows 140, may leak or escape and propagate across the surface of the substrate. An acoustic wave propagating across the surface of the substrate may reflect at the edges of the substrate. Additionally, since the velocity of an acoustic wave is different between regions of the substrate that are and are not covered by conductors, a portion of the energy of an acoustic wave will reflect each time the acoustic wave encounters the edge of a conductor.
SAW resonators are used in a variety of radio frequency filters including band reject filters, band pass filters, and duplexers. A duplexer is a radio frequency filter device that allows simultaneous transmission in a first frequency band and reception in a second frequency band (different from the first frequency band) using a common antenna. Duplexers are commonly found in radio communications equipment including cellular telephones.
Filter circuits commonly incorporate more than one SAW resonator. For example, FIG. 2 shows a schematic diagram of a filter circuit 200 incorporating nine SAW resonators, labeled Xa through Xi. The use of nine SAW resonators is exemplary and a filter circuit may include more or fewer than nine SAW resonators. The filter circuit 200 may be, for example, a band pass filter, a band reject filter, or a combination band pass/band reject filter depending on the characteristics of the SAW resonators.
The nine SAW resonators Xa through Xi are typically fabricated in close proximity on a common substrate. Since the SAW resonators are in close proximity, acoustic energy that leaks from a first resonator may impinge upon one or more other resonators, either directly or after reflection from an edge of the substrate or an edge of a conductor pattern. The one or more other resonators that receive the leaked acoustic energy may convert some or all of the leaked acoustic energy into electrical signals. For example, acoustic energy leaking from SAW resonator Xa may impinge on SAW resonator Xg, as indicated by the dashed arrow 210, and acoustic energy leaking from SAW resonator Xb may impinge on SAW resonator Xf, as indicated by the dashed arrow 220. Leaked acoustic energy may effectively provide sneak paths by which RF signals can bypass portions of the filter circuit.
FIG. 3 shows a graph 300 comparing the simulated and measured performance of a combination band pass/band reject filter circuit similar to the filter circuit 200 shown in FIG. 2. The graph 300 plots |S(1,2)| (the magnitude in dB of the transfer function between port 1 and port 2 of the filter) as a function of frequency. The solid line 310 is the expected filter performance based on electromagnetic modeling of the filter circuit. The dashed line 320 is the measured performance of a prototype filter. The measured transfer function (dashed line) closely approximates the modeled performance (solid line) over the band pass region centered at 1.785 GHz. The measured transfer function (dashed line) deviates substantially (i.e. as much as 18 dB) from the modeled performance (solid line) over the frequency band from 1.805 GHz to 1.85 GHz. The unexpectedly low insertion loss of the actual filter in this frequency band may result, at least in part, from acoustic leakage paths that are not included in the electromagnetic modeling.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.